You can use a two-handed backhand on your high backhand overhead...by letting the ball drow to head or shoulder level.

Often, however, you will not have the time or the ball isn't one you can let drop. You will need to work on your true backhand overhead, a shot hit by turning sideways and getting your racquet up early. Make sure you are using a continental grip and that you get your elbow high on the backswing. You can either cantilever your racquet over the elbow and square the racquet up as if giving a reverse 'high five'...(A high five with the back of your hand.) There is some supination as the forearm rotates away from the body allowing the racquet to square up...sort of the opposite of pronation you do on your serve. This creates good racquet head speed.

Avoid trying to swing with the whole arm as this slows the racquet head down and makes for a weak swing...even as swinging with the arm 'feels' like you are going to have more strength.

Finally, hold your arm up after contact. This will help keep from swinging with the arm and allow the racquet at the point of the last link in the kinetic chain to be accelerated.

If you can, the best thing to do is to run around and hit a regular overhead. It is easier and will most likely end the point anyway. For the shots that you can't run around, I am afraid you will need to learn a backhand overhead.